1. Field of the Invention
The invention relates to data compression systems based on the LZ data compression methodology and more particularly on the LZW protocols.
2. Description of the Prior Art
Professors Abraham Lempel and Jacob Ziv provided the theoretical basis for LZ data compression and decompression systems that are in present day widespread usage. Two of their seminal papers appear in the IEEE Transactions on Information Theory, IT-23-3, May 1977, pp. 337-343 and in the IEEE Transactions on Information Theory, IT-24-5, Sep. 1978, pp. 530-536. A ubiquitously used data compression and decompression system known as LZW is described in U.S. Pat. No. 4,558,302 by Welch, issued Dec. 10, 1985. LZW has been adopted as the compression and decompression standard used in the GIF image communication protocol and is utilized in the TIFF image communication protocol. GIF is a development of CompuServe Incorporated and the name GIF is a Service Mark thereof. A reference to the GIF specification is found in GRAPHICS INTERCHANGE FORMAT, Version 89a, Jul. 31, 1990. TIFF is a development of Aldus Corporation and the name TIFF is a Trademark thereof. Reference to the TIFF specification is found in TIFF, Revision 6.0, Finalxe2x80x94Jun. 3, 1992.
LZW has also been adopted as the standard for V.42 bis modem compression and decompression. A reference to the V.42 bis standard is found in CCITT Recommendation V.42 bis, Data Compression Procedures For Data Circuit Terminating Equipment (DCE) Using Error Correction Procedures, Geneva 1990. The V.42 bis standard is further described in an article entitled xe2x80x9cV.42 bis: The New Modem Compression Standardxe2x80x9d by J. E. MacCrisken in the Spring 1991 issue of the Journal Of Data and Computer Communicationsxe2x80x94Modem Compression, pages 23-29.
Examples of LZ dictionary based compression and decompression systems are described in the following U.S. patents: U.S Pat. No. 4,464,650 by Eastman et al., issued Aug. 7, 1984; U.S. Pat. No. 4,814,746 by Miller et al., issued Mar. 21, 1989; U.S. Pat. No. 4,876,541 by Storer, issued Oct. 24, 1989; U.S. Pat. No. 5,153,591 by Clark, issued Oct. 6, 1992; U.S. Pat. No. 5,373,290 by Lempel et al., issued Dec. 13, 1994; U.S. Pat. No. 5,838,264 by Cooper, issued Nov. 17, 1998; U.S. Pat. No. 5,861,827 by Welch et al., issued Jan. 19, 1999; and U.S. Pat. No. 6,320,523 by York et al., issued Nov. 20, 2001.
In the above dictionary based LZ compression and decompression systems, the compressor and decompressor dictionaries may be initialized with all of the single character strings of the character alphabet. In some implementations, the single character strings are considered as recognized and matched although not explicitly stored. In such systems the value of the single character may be utilized as its code and the first available code utilized for multiple character strings would have a value greater than the single character values. In this way the decompressor can distinguish between a single character string and a multiple character string and recover the characters thereof. For example, in the ASCII environment, the alphabet has an 8 bit character size supporting an alphabet of 256 characters. Thus, the characters have values of 0-255. The first available multiple character string code can, for example, be 258 where the codes 256 and 257 are utilized as control codes as is well known.
In the prior art dictionary based LZ compression systems, data character strings are stored and accessed in the compressor dictionary utilizing well known searchtree architectures and protocols. Typically, the searchtree is arranged in nodes where each node represents a character, and a string of characters is represented by a node-to-node path through the tree. When the input character stream has been matched in the dictionary tree up to a matched node, a next input character is fetched to determine if the string match will continue. Conventionally, a determination is made to ascertain if the fetched character is already stored as an extension node of the matched node. Various techniques are utilized to effect this determination such as associative memory dictionaries, hashing and sibling lists as are well understood in the art.
In the above dictionary based systems, numerous dictionary accesses are required at the compressor for compressing an input stream of data characters. Normally several dictionary accesses are required for each input data character and when utilizing an associative memory, it may be necessary to search the entire memory to determine if a string exists therein. It is desirable in such systems to minimize the number of dictionary accesses so as to enhance system performance.
Although the known dictionary architecture and protocols provide efficient data compression systems, it is a continuing objective in the art to improve compressor performance.
The present invention replaces the conventional dictionary arrangements with digital logic elements and switches to provide a new architecture and protocols which, it is believed, will improve the performance of LZ type data compression systems.
The present invention is embodied in a data compressor for compressing an input stream of data characters into an output stream of compressed codes. The compressor includes a plurality of coincidence elements corresponding to a respective plurality of strings, the coincidence elements having a respective plurality of coincidence outputs. A string is comprised of a prefix string of at least one of the data characters followed by an extension character, a prefix string having a prefix code associated therewith. A coincidence element includes inputs responsive, respectively, to a representation of a predetermined prefix code and a representation of a predetermined character for energizing the coincidence output thereof upon coincidental energization of the inputs.
A plurality of codes are assigned to selected coincidence elements, respectively, so that energization of a coincidence output of a selected coincidence element provides a representation of the code assigned thereto. The provided representations of codes assigned to the coincidence elements are coupled to the prefix code inputs of the coincidence elements and representations of data characters fetched from the input stream are coupled to the character inputs of the coincidence elements.
Data characters are sequentially fetched from the input stream so as to sequentially energize coincidence outputs until a last coincidence output is energized of a coincidence element that does not have a code assigned thereto. The code is output that is assigned to the coincidence element having the coincidence output energized prior to the last coincidence output so as to provide the stream of compressed codes. A next assignable code is assigned to the coincidence element having the last energized coincidence output.
Specifically, in the preferred embodiments, a switch arrangement, such as a matrix switch, is included having a plurality of switch inputs and a plurality of switch outputs, the plurality of coincidence outputs being coupled to the plurality of switch inputs, respectively. The switch is controllably operative for coupling any one of the switch inputs to a selected one of the switch outputs, so that the coincidence outputs are selectively coupled to the switch outputs. Code assignment elements are included for assigning the plurality of codes to the plurality of switch outputs, respectively, so that energization of a coincidence output coupled to a switch output provides a representation of the code assigned to the switch output.
The provided representations of codes assigned to the switch outputs are coupled to the prefix code inputs of the coincidence elements and representations of data characters fetched from the input stream are coupled to the character inputs of the coincidence elements.
Data characters are sequentially fetched from the input stream so as to sequentially energize switch outputs until a coincidence output is energized that is not coupled to a switch output. The code assigned to the last energized switch output is outputted to provide the stream of compressed codes. The switch is controlled to selectively couple the last energized coincidence output to the switch output corresponding to the next assignable code.
In the preferred embodiments, a representation of the fetched data character that resulted in energizing the last coincidence output is applied to the prefix code inputs of the coincidence elements to begin a next compression cycle.